1. Field of the Invention
The present invention relates to a digital fluoroscopic apparatus, and more particularly to a digital subtraction-imaging apparatus wherein digital subtraction between mask images and contrast images is performed in synchronism with heartbeats.
2. Description of Related Art
In a known digital fluoroscopic apparatus, in order to acquire a plurality of X-ray images and perform image processing, bone or muscle image components are erased from an image, by subtraction between an image (mask image) taken before an X-ray contrast medium is injected into a object under medical examination such as a patient, and an image (contrast image) taken after the injection of an X-ray contrast medium, and only an image component reflecting the X-ray contrast medium can be obtained as an image.
A cardiac-synchronized subtraction method is available for use in a digital fluoroscopic apparatus of this type. According to this method, in order to examine a patient's heart, mask images corresponding to one cardiac cycle are acquired, and subtraction is performed between these mask images and contrast images of the same cardiac beat phase, thereby removing image components due to the cardiac beat from the subtraction image.
This known method will now be described with reference to FIGS. 1, 2A, and 2B.
First, mask images are acquired for one cardiac period ("t.sub.0 " in FIG. 1), at a rate of 30 frames/sec, i.e., for about 30 frames at a rate of 60 heartbeats/min, as shown in FIG. 1. In FIG. 1, mask images are acquired only at 7 frames/sec, for the purpose of simplicity. An X-ray contrast medium is injected into the object under medical examination, and a predetermined number of contrast images are acquired at respective cardiac periods t1, t2, t3, . . . , tn. Then, subtraction is performed successively between mask images M1 to M7 and contrast images C1 to Cn of the same cardiac beat phase, as shown in FIG. 2A. Subsequently, as shown in FIG. 2B, subtraction is performed successively between mask images M1 to M7 and contrast images C8 to C14 of a second heartbeat, as shown in FIG. 2B. In this manner, the same mask images M1 to M7 are selectively used, and subtraction is performed successively between these mask images M1 to M7 and corresponding contrast images C1 to Cn. The cardiac beat phases of the mask and contrast images are allowed to coincide, i.e., are subjected to contrast synchronization by use of an electrocardiogram.
This conventional cardiac-synchronized subtraction method is known from, for example, "EKG-Gated Digital Subtraction Angiography in the Detection of Pulmonary Emboli.sup.1 " by Mohamed Hirji et al. published in 1984 "Radiology", 152; Pages 19-22.
In such a cardiac subtraction process, when the heart rate remains constant during image acquisition, no problems occur, since the number of mask images always coincides with that of contrast images acquired during each cardiac period. However, the heart rate can change. In particular, when an X-ray contrast medium is injected, the cardiac beat rate can be temporarily disturbed. More specifically, variations in heartrate (i.e., the number of heartbeats) cause variations in respective cardiac periods t1 to tn. As a result, the number of the contrast images acquired during each cardiac period may increase or decrease. In this situation, desired subtraction between mask and contrast images of the same cardiac beat phase cannot be performed. In other words, when subtraction is performed between mask and contrast images of different cardiac beat phases (i.e., when non cardiac-synchronized subtraction is performed), an artifact might appear in the obtained subtraction images.
The present invention has been in view of the above situation, and has as its object to provide a cardiacsynchronized subtraction system which can perform subtraction, without interruption, by reliably allowing cardiac beat phases to coincide even when the cardiac beat rhythm is disturbed or the number of heart beats varies.